Air gap extending the width of a permanent magnet assembly

ABSTRACT

A magnet assembly includes a permanent magnet having a pair of steel plates one fastened to each of its top and bottom sides. In one embodiment the magnet and plates are tubular with the inside diameter of the magnet being substantially greater than the inside diameter of the plates. A steel outer pole piece is secured to the inside opening of the plates in spaced relationship to a steel inner pole piece so as to provide an air gap therebetween. In another embodiment the plates are solid with the plates having a substantially greater diameter than the magnet. An inner steel pole piece is secured to the outer periphery of the plates in spaced relationship to an outer steel pole piece so as to provide an air gap therebetween. In each embodiment a coil is received in the air gap.

Tl'ite States atet [191 Rooney [75] Inventor: Raymond 1R. Rooney, Crystal Lake,

[73] Assignee: The Arnold Engineering Company,

Chicago, Ill.

[22] Filed: May 11, 1973 [21] Appl. No.: 359,369

[52] US. C11 335/231, 179/119, 179/120 [51] Int. G11. H011 7/00 [58] Field0fSearch335/231; 179/115.5, 115.5 DV,

179/l15.5 SF,117, 119,120

[56] References Cited UNITED STATES PATENTS 2,522,530 9/1950 Miner 179/120 3,073,411 1/1963 Bleazey et a1 179/115.5 R

FOREIGN PATENTS OR APPLICATIONS 1,361,069 6/1963 France 335/231 858250 1/1961 Great Britain 335/231 Primary ExaminerGeorge Harris Attorney, Agent, or Firm-Vincent G. Gioia [57] ABSTRACT A magnet assembly includes a permanent magnet having a pair of steel plates one fastened to each of its top and bottom sides. In one embodiment the magnet and plates are tubular with the inside diameter of the magnet being substantially greater than the inside diameter of the plates. A steel outer pole piece is secured to the inside opening of the plates in spaced relationship to a steel inner pole piece so as to provide an air gap therebetween. In another embodiment the plates are solid with the plates having a substantially greater diameter than the magnet. An inner steel pole piece is I secured to the outer periphery of the plates in spaced relationship to an outer steel pole piece so as to provide an air gap therebetween. In each embodiment a coil is received in the air gap.

16 Claims, 3 Drawing Figures MUENTEMPR 9 .974

PIG- B AIR GAP EXTENDING 'IIIE WID'III OF A PERMANENT MAGNET ASSEMBLY This invention relates to a permanent magnet assembly particularlyfor use in moving coil assemblies such as loud speakers and force motors. In such assemblies a magnetic field is produced in an air gap for interaction with a current carrying coil which is movable in the air gap. The magnetic assemblies of which I have knowledge have single air gaps so that the efficiency is limited.

It is, therefore, an object of my invention to provide permanent magnet assemblies having greater efficiencies than prior assemblies.

The foregoing and other objects of the invention will be best understood from the following description, reference being had to the accompanying drawing wherein:

FIG. I is a sectional view of a magnet assembly of my invention;

FIG. 2 is an elevation of a coil used with the magnet assemblies and showing its relationship with the assembly; and

FIG. 3 is a view, similar to FIG. I, showing a second embodiment of my invention.

Referring more particularly to FIG. 1, reference numeral 2 indicates a permanent magnet having top and bottom sides 4 and 6. It will be understood that the terms top and bottom are used for convenience only since the assembly may'be used in any position. The magnet 2 is tubular with an inner opening 8. Tubular plates I having an inner opening I2 are secured to the top and bottom sides of magnet 2 in any suitable manner such as by means of an adhesive. A tubular outer pole piece 14 is secured in openings I2 such as by an adhesive with its ends flush with the outside of the plates 10. An inner pole piece I6 is positioned within pole piece I4 and has an outer diameter less than the inside diameter of pole piece I4 so as to provide an air gap 18. A central slot may be provided around the outside periphery of pole piece I4. A plate 22 attached to pole piece 16 and bottom plate It) in any suitable manner positions pole piece I6 in a central location. The parts may be connected by an adhesive or as shown the plate 22 is secured to pole piece 16 by means of screw 24 screwed into threaded opening 26 in pole piece I6.

The magnet 2 is magnetized parallel to its length. The plates I0 and pole pieces 14 and 16 are made of magnetizable material such as ferromagnetic material. Steel is preferred. The plate 22 and preferably also screw 24 are made of non-magnetizable material such as brass. In one particular assembly the magnet 2 has a height or thickness of 0.425 in., an outside diameter of 3.375

in. and an inside diameter of 1.500 in.; the plates I0 have a thickness of 0.250 in., an outside diameter of 3.125 in., and an inside diameter of 1.242 i 0.001 in.; the pole piece 12 has a length of 0.925 in., an outside diameter of 1.242: 0.001 in. so as to have a slip. fit with plates 10, an inside diameter of 1.062 in., a slot width of 0.125 in. and a wall thickness of 0.030 in. at the bottom of the slot; and the pole piece I6 has a length of 0.925 in. and a diameter of 0.986 1' 0.001 in.

A tubular coil assembly 28 is received in air gap 18 for movement with respect thereto. As shown in FIG. 2 the coil assembly 28 includes a tube 30 made of any suitable non-conducting material with a copper wire coil 32 wound thereon in opposite directions from its center. The coil 32 is connected to a suitable power source LI, L2. The plate 22 may have a groove '34 in its inner face to accommodate movement of coil assembly 28. The distance between the pole piece 14 and coil assembly 28 is exaggerated for the purpose of illustration.

It will be seen that the magnetic circuit flux path is through one end plate I0, through one end of pole piece 14, across the air gap I8, through pole piece I6, across the air gap I8, through the other end of pole piece I4, and through the other end plate It) back to the magnet. The wall thickness of pole piece 14 should be selected in a well known manner to be sufficiently thick to provide a uniform gap flux density throughout a desired axial gap length and to be sufficiently thin to prevent an unnecessary amount of flux passing through it in an axial direction rather than across the air gap. The slot 20 is used for this purpose since it permits only a small percentage of the magnet flux through the pole piece.

The magnetic circuit efficiency, or ratio of magnet flux to gap flux, is approximately the same in each of the two air gap flux paths as in the single air gap flux paths of present magnet assemblies. Thus, the cumulative effect of these two air gap flux paths in series is approximately double that of the prior art so that the effect of a magnetic efficiency of near percent may be obtained.

The reluctance factor, or ratio of magneto motive force available in the magnet to the magneto motive force supplied to the air gap is approximately the same as in a conventional assembly but since there are two air gap flux paths in series, the magneto motive force required of the magnet to supply a specific air gap flux density in a specific air gap length is greater than in conventional assemblies. Precise dimensions of the parts for maximum efficiency can be determined by well known principals. For example the distance between the magnet 2 and pole piece 14 is a compromise between the desire to have a long distance to minimize leakage and the desire to have compactness. The distance must be substantially greater than the width of the air gap.

In the specific assembly described above, the total flux change in the coil is 46,200 maxwells and the total flux produced by the magnet is 113,700 maxwells. Thus, the leakage factor is 113,700 (2 X 46,200) or 1.23 whereas that in a conventional speaker magnet is nearly 2.0.

In the species shown in FIG. 3, reference numeral 35 indicates a permanent magnet having plates 36 secured to its top and bottom sides. The magnet 35 is magnetized parallel to its length. A tubular inner pole piece 38 is secured to the outer periphery of plates 36 and is provided with a central slot 40 around its inner periphery. A tubular outer pole piece 42 having an inside diameter greater than the outside diameter of the pole pice 38 surrounds the pole piece 38 in spaced relationship to provide an air gap 44. A non-magnetic, e.g., brass, plate 46 fastened to bottom plate 36 and pole pieces 38 and 42 hold the parts in position. Like plate 22, the plate 46 is preferably provided with a groove 48 in its upper surface to receive coil assembly 28. The various parts are held together with a suitable adhesive. The various parts are made of the same materials as the corresponding parts of FIG. 1 and the operation is essentially the same as that of FIG. ll.

For convenience and ease of manufacture it is desirable that the various parts be cylindrical, but it will be understood that they could have various other shapes. For that reason the term diameter is used to indicate length and/or widths of shapes other than circular.

While several embodiments have been described, it will be readily apparent to those skilled in the art that various changes may be made to suit requirements without departing from the spirit and scope of the invention.

I claim:

1. A magnet assembly comprising a permanent magnet having top and bottom sides, said magnet being magnetized parallel to its length, a pair of plates of magnetizable material one secured to each of said sides, an inner pole piece of magnetizable material, a tubular outer pole piece of magnetizable material, the outer diameter of said inner pole piece being less than the inner diameter of said outer pole piece, one of said pole pieces extending between and secured to said plates in spaced relationship to said magnet, said pole pieces extending beyond said top and bottom sides, and means holding said inner and outer pole pieces in spaced apart relationship to provide an air gap of substantially uniform width around the periphery thereof extending beyond said top and bottom sides whereby the flux created by said magnet passes through said air gap beyond said top and bottom sides in series.

2. An assembly according to claim 1 including a tubular coil mounted in said air gap for relative axial movement with respect to said pole pieces.

3. An assembly according to claim 1 in which said means holding said inner and outer pole pieces in spaced apart relationship includes a non-magnetizable plate secured to one of said first named plates and to said pole pieces.

4. An assembly according to claim 3 including a tubular coil mounted in said air gap for relative axial movement with respect to said pole pieces.

5. An assembly according to claim 1 in which said magnet, plates and pole pieces are separate coaxial cylinders fastened together and the ends of said pole pieces extend to the outer side of said plates, said plates and pole pieces being made of ferromagnetic material.

6. An assembly according to claim 5 including a tubular coil mounted in said air gap for relative axial movement with respect to said pole pieces.

7. An assembly according to claim 6 in which said means holding said inner and outer pole pieces in spaced apart relationship includes a non-magnetizable plate secured to one of said first named plates and to said pole pieces.

8. An assembly according to claim 7 in which said coil incudes a tube of non-magnetizable material, and a coil of electrical conducting material on said tube wound in opposite directions from its center.

9. An assembly according to claim 1 in which said permanent magnet and said plates are tubular in shape, the inner diameter of said magnet being substantially greater than the outer diameter of said outer pole piece so as to provide a space therebetween substantially greater than the width of said air gap, said outer pole piece being secured to the inside openings of said plates.

10. An assembly according to claim 9 in which said outer pole piece has a central slot around its outside periphery.

11. An assembly according to claim 10 in which said means holding said inner and outer pole pieces in spaced apart relationship includes a non-magnetizable plate secured to one of said first named plates and to said pole pieces.

12. An assembly according to claim 11 including a tubular coil mounted in said air gap for relative axial movement with respect to said pole piece.

13. An assembly according to claim 1 in which said inner pole piece is tubular in shape, the diameter of said plates being substantially greater than the diameter of said magnet, said plates being secured to the inside of said inner pole piece.

14. An assembly according to claim 13 in which said inner pole piece has a central slot around its inside periphery.

15. An assembly according to claim 14 in which said means holding said inner and outer pole pieces in spaced apart relationship includes a non-magnetizable plate secured to one of said first named plates and to said pole pieces.

16. An assembly according to claim 15 including a tubular coil mounted in said air gap for relative axial movement with respect to said pole pieces. 

1. A magnet assembly comprising a permanent magnet having top and bottom sides, said magnet being magnetized parallel to its length, a pair of plates of magnetizable material one secured to each of said sides, an inner pole piece of magnetizable material, a tubular outer pole piece of magnetizable material, the outer diameter of said inner pole piece being less than the inner diameter of said outer pole piece, one of said pole pieces extending between and secured to said plates in spaced relationship to said magnet, said pole pieces extending beyond said top and bottom sides, and means holding said inner and outer pole pieces in spaced apart relationship to provide an air gap of substantially uniform width around the periphery thereof extending beyond said top and bottom sides whereby the flux created by said magnet passes through said air gap beyond said top and bottom sides in series.
 2. An assembly according to claim 1 including a tubular coil mounted in said air gap for relative axial movement with respect to said pole pieces.
 3. An assembly according to claim 1 in which said means holding said inner and outer pole pieces in spaced apart relationship includes a non-magnetizable plate secured to one of said first named plates and to said pole pieces.
 4. An assembly according to claim 3 including a tubular coil mounted in said air gap for relative axial movement with respect to said pole pieces.
 5. An assembly according to claim 1 in which said magnet, plates and pole pieces are separate coaxial cylinders fastened together and the ends of said pole pieces extend to the outer side of said plates, said plates and pole pieces being made of ferromagnetic material.
 6. An assembly according to claim 5 including a tubular coil mounted in said air gap for relative axial movement with respect to said pole pieces.
 7. An assembly according to claim 6 in which said means holding said inner and outer pole pieces in spaced apart relationship includes a non-magnetizable plate secured to one of said first named plates and to said pole pieces.
 8. An assembly according to claim 7 in which said coil incudes a tube of non-magnetizable material, and a coil of electrical conducting material on said tube wound in opposite directions from its center.
 9. An assembly according to claim 1 in which said permanent magnet and said plates are tubular in shape, the inner diameter of said magnet being substantially greater than the outer diameter of said outer pole piece so as to provide a space therebetween substantially greater than the width of said air gap, said outer pole piece being secured to the inside openings of said plates.
 10. An assembly according to claim 9 in which said outer pole piece has a central slot around its outside periphery.
 11. An assembly according to claim 10 in which said means holding said inner and outer pole pieces in spaced apart relationship includes a non-magnetizable plate secured to one of said first named plates and to said pole pieces.
 12. An assembly according to claim 11 including a tubular coil mounted in said air gap for relative axial movement with respect to said pole piece.
 13. An assembly according to claim 1 in which said inner pole piece is tubular in shape, the diameter of said plates being substantially greater thaN the diameter of said magnet, said plates being secured to the inside of said inner pole piece.
 14. An assembly according to claim 13 in which said inner pole piece has a central slot around its inside periphery.
 15. An assembly according to claim 14 in which said means holding said inner and outer pole pieces in spaced apart relationship includes a non-magnetizable plate secured to one of said first named plates and to said pole pieces.
 16. An assembly according to claim 15 including a tubular coil mounted in said air gap for relative axial movement with respect to said pole pieces. 